How to control the direction for the Build modifier?
The Build Modifier will use the vertices index number to determine their apparition order. These indices are determined by their order of creation. In your case, if the Build modifier displays the top, then the bottom, then the rest, it's probably because you've created the horizontal edge loops with a bevel operation. With successive extrusions the order will have gone from top to bottom.
To display the Indices option in 2.9, enable Preferences &gt; Interface &gt; Developer Extras, then go in Edit Mode, select the vertices and enable the Indices option in the  Viewport Overlays panel.
To sort the vertices the way you want, go in Edit Mode, put the 3D Cursor where you want, it will determine the last numbers, select all, in Mesh &gt; Sort Elements &gt; Cursor Distance, then in the Operator box click on the Face button.
For more complicated objects than simple vertical/horizontal ones there must be some addons or python codes that allow to sort the vertices index numbers.
You can play with the Build modifier Start value, if you want the object to appear in the other direction, click on Reversed.